1. Field of the Invention
The present invention relates to a monitoring camera and a monitoring method of a monitoring camera system, and more particularly, to a technique suitably used to obtain a high-quality image by a monitoring camera.
2. Description of the Related Art
A monitoring camera system for transmitting video data captured by a camera via a network and recording the video data in a storage device such as a hard disk has now spread widely. In this monitoring camera system, it is essential to compress the video data captured by the camera to efficiently use a finite network band and the storage capacity of the storage device. Generally, a video coding device for compressing video data is mounted in each camera.
A video coding system has been currently standardized by International Telecommunication Union Telecommunication Standardization Sector (ITU-T), which is an international standards organization. In monitoring cameras, H. 263 and newest H. 264, standardized by ITU-T, have been adopted as the video coding system.
Both the general video coding systems employ motion compensation (interframe prediction) using a motion vector, and enable a code amount to be efficiently reduced using correlation between continuous frames.
FIG. 10 illustrates an example of the internal configuration of a monitoring camera including a video coding device.
As illustrated in FIG. 10, the monitoring camera includes an image sensor 1001 such as a charge coupled device (CCD) sensor or a complementary metal oxide semiconductor (CMOS) sensor, a development device 1002, and a video coding device 1003. The developing device 1002 converts video information, which has been converted into an electrical signal by the image sensor 1001, into video information suitable for viewing by a person. The video coding device 1003 codes the input video information, and sends the coded video information as a video stream onto a network or to a storage device or the like.
In recent years, a monitoring camera system for performing multifaceted monitoring of one scene using a plurality of cameras has started to be put into practical use to increase the reliability and the accuracy of the monitoring. The use of the plurality of cameras can prevent situations in which only a back surface of a suspicious person remains as a video so that the feature of the suspicious person cannot be extracted, for example.
FIG. 9 illustrates an example of a monitoring camera system using a plurality of cameras. As illustrated in FIG. 9, in the monitoring camera system using the plurality of cameras, the plurality of cameras simultaneously captures the same area. When a suspicious person intrudes into the area, for example, motions occur simultaneously in scenes respectively captured by the cameras.
In the above-mentioned video coding system, when motions occur in scenes, correlation between continuous frames relatively decreases then cause increase of the bit rate of video stream to keep the same image quality. Particularly at sudden occurrence of complicated motion unlike horizontal/vertical motion, which is not easily represented by a motion vector, the increase of bit rate of the video stream becomes inevitable.
FIG. 12 illustrates an example of the transition of a bit rate when a motion occurs in a scene in the monitoring camera system including three monitoring cameras A, B, and C illustrated in FIG. 9.
As illustrated in FIG. 12, when the motion occurs in the scene, the bit rate of a video captured by any of the monitoring cameras increases. Such a rapid increase in the bit rate results in failing to write to a storage device whose writing speed has its upper limit or transmission delay during network transmission.
In order to prevent the bit rate from those rapid increasing, a quantization parameter is generally controlled to make an output bit rate constant in a video coding device. Such bit rate control is referred to as constant bit rate (CBR) control. In the CBR control, when a motion occurs, the quantization parameter is increased to keep the bit rate constant.
When the quantization parameter increases, however, an image quality in a scene where a motion occurs degrades even though the scene is important for a monitoring application. Therefore, the feature of the suspicious person is not easy to specify and could be problematic.
Japanese Patent Laid-Open No. 2008-11324, for example, discusses a method for preventing a bit rate from rapidly increasing when a motion occurs. According to the method, a change of a scene is detected from information during coding, and video data is input to a video coding device after its high-frequency component is previously reduced by a filter when the scene changes.
Japanese Patent Laid-Open No. 2007-243699, for example, discusses that in a situation in which a plurality of cameras perform image-capturing, the frame rate of the camera in which an event occurs is changed. Further, the frame rate of the other cameras can also be changed. Therefore, the frame rate is controlled in corporation among the cameras.
As described above, when the motion occurs in the scene in the monitoring camera system using the plurality of cameras, the image quality degrades even though the scene is important for the monitoring application. Therefore, the feature of the suspicious person is not easy to specify.
In the method discussed in Japanese Patent Laid-Open No. 2008-11324, described above, the high-frequency component is uniformly reduced using the filter when the scene changes due to the occurrence of the motion. Since the increase in the bit rate is not monitored, filtering is performed although it is not required. As a result, the high-frequency component unnecessarily degraded then cause generation of blurred image.
In the method discussed in Japanese Patent Laid-Open No. 2007-243699, described above, the frame rate may be enhanced in a scene where no motion occurs. Generally in a monitoring application, the feature of a suspicious person is specified using one frame in video sequences in many cases. Therefore, it is more important to enhance image quality rather than improving the frame rate.